1. Field of the Invention
This invention relates to a pneumatic tire comprising a bead core formed by laminating three or more sheath layers on a core wire.
2. Description of Related Art
In pneumatic tires used under conditions of a high speed and a heavy load such as a pneumatic tire for an aircraft and the like, a large stress is usually applied to a bead portion during the running, so that a bead core durable to such a large stress must be embedded in the bead portion.
As the bead core durable to such a large stress, there has been known, for example, a bead core as disclosed in JP-A-53-51804, that is, a bead core 1 called as a cable bead formed by laminating plural sheath layers 4, each of which layers being constituted by substantially spirally twisting a plurality of fine sheath filaments 3, on a single thick core wire 2 as shown in FIG. 5.
In the conventional bead core 1, the twisting direction of the sheath filament 3 in the sheath layers 4 becomes reverse every the sheath layer. For example, when the sheath filament 3 in a certain sheath layer 4 among the sheath layers 4 is Z-lay, the sheath filaments in both sheath layers adjacent thereto (inside and outside sheath layers) are S-lay. When the twisting direction of the sheath filament 3 is reverse every the sheath layer as mentioned above, the torsional rigidity of the bead core 1 can be increased, whereby the change of deformation bearing ratio in each tire portion can be decreased in correspondence with the change of slip angle to improve steering stability and high-speed straight running performance.
However, when the pneumatic tire provided with the conventional bead core 1 is used over a long period of time, there is caused a problem that the fatigue fracture is often created in the sheath filament 3 of a second sheath layer 4 viewed from the outermost side to degrade the durability of the bead portion.
This is due to the fact as mentioned below. That is, when the pneumatic tire is run under a heavy load, the bead core 1 is repeatedly subjected to torsion in left-handed and right-handed rotations under an action of a large external force. In this case, if the torsion in the right-handed rotation (clockwise rotation) is applied to the bead core 1, a sheath layer 4Z in which a twisting direction of a sheath filament 3 is Z-lay is deformed into a direction of opening the sheath filaments 3Z (or a direction of increasing the size of the sheath layer 4Z), while a sheath layer 4S in which a twisting direction of a sheath filament 3 is S-lay is deformed into a direction of closing the sheath filaments 3S (or a direction of decreasing the size of the sheath layer 4S). On the other hand, if the torsion in the left-handed rotation (anticlockwise rotation) is applied to the bead core 1, the sheath layer 4Z of Z-lay is deformed into the direction of closing the sheath filaments 3Z (or the size-decreasing direction), while the sheath layer 4S of S-lay is deformed into the direction of opening the sheath filaments 3Z (or the size-increasing direction).
As a result, if sheath filaments in a certain sheath layer are Z-lay and sheath filaments in both sheath layers adjacent to the Z-lay sheath layer are S-lay as mentioned above, when the torsion in the right-handed rotation is applied to the bead core, the sheath filaments 3Z in the Z-lay sheath layer 4Z deforming into the size-increasing direction directly contact with the sheath filaments 3S of the outer S-lay sheath layer 4S deforming into the size-decreasing direction to rub them with each other and also the sheath filaments 3Z are pushed down by the outer sheath filaments 3S to locally dent at the contacting position inward in the radial direction. On the other hand, when the torsion in the left-handed rotation is applied to the bead core, the sheath filaments 3Z in the Z-lay sheath layer 4Z deforming into the size-decreasing direction directly contact with the sheath filaments 3S of the inner S-lay sheath layer 4S deforming into the size-increasing direction to rub them with each other and also the sheath filaments 3S are pushed down by the outer sheath filaments 3Z to locally dent at the contacting position inward in the radial direction.
The rubbing force and the dent deformation amount as mentioned above become larger at more outer sheath layer among the sheath layers. However, the sheath filaments in the outermost sheath layer are not rubbed or dented by pushing in the deformation into the size-increasing direction. Therefore, it is frequent that the fatigue fracture is firstly created in the sheath filaments of a second sheath layer viewed from the outermost side of all sheath layers by the rubbing and dent deformation as mentioned above.